A simple theory for the dependence of the electrical resistance of the magnetic superconductors TmRh4B4 and ErRh4B4 on temperature and field

The warmup rate of the¹⁶⁵Ho-enhanced nuclear spins in a HoVO₄ single crystal in contact with dilute³He is shown to be dominated by the Kapitza resistanceR_K at low bath temperature (T_b<100 mK). This result is obtained from an analysis of the NMR signal during the warmup, following a demagnetization which cools the sample to a temperature of about 5 mK. The temperature variation ofR_K (T_b<^/–3) is consistent with the acoustic mismatch model, but the magnitude is 40 times smaller than the theoretical predictions for a solid-liquid interface. The spin-lattice relaxation is also estimated, and found to be in agreement with the results of other authors.